In typical iron ore gaseous reduction systems incorporating a vertical shaft, moving bed reactor as exemplified in U.S. Pat. Nos. 3,765,872; 3,779,341 and 4,216,011, the iron ore is reduced by contacting it with a reducing gas having a relatively high reducing potential and a correspondingly low oxidant concentration. Such direct reduction systems utilize a vertical shaft, moving bed reactor having a reduction zone in the upper portion thereof and a cooling zone in the lower portion thereof. The iron ore is introduced through the top of the reactor and caused to flow downwardly through the reducing zone in which the ore is constructed with a heated reducing gas largely composed of carbon monoxide and hydrogen. The ore which has been reduced in the reducing zone flows into and downwardly through the cooling zone in which it is controllably cooled and carburized by a gaseous coolant prior to being discharged through the bottom of the reactor. The spent reducing gas leaving the reduction zone of the reactor is de-watered in a quench cooler and, if desired, is further upgraded by the removal of carbon dioxide. A major portion of this cooled, upgraded gas stream is then reheated and recycled to the reduction zone of the reactor to form a reducing gas loop. Similarly, a portion of the cooling gas is withdrawn from the cooling zone of the reactor, cooled and recycled to the cooling zone to form a cooling loop.
The reducing gas fed to the reduction zone of the reactor is typically generated in a conventional, catalytic reforming unit charged with steam and a suitable hydrocarbon-containing gas. In reduction systems utilizing a conventional reformer, before the reformed gas can be fed to the reactor as a reducing gas it must be de-watered to avoid the undesirable accumulation of excess oxidants (namely, carbon dioxide and water) in the reducing gas. The concentration of oxidants can be controlled by feeding the reformed gas to a quench cooler by which water is removed and then re-reheating the gas to the desired reduction temperature prior to being fed to the reactor.
In certain other known reduction systems such as those exemplified in U.S. Pat. Nos. 3,617,227; 3,748,120; 3,749,386; 3,764,123 and 3,905,806, the reducing gas generated in the reformer can be fed directly to the reduction reactor without removing water from the reformed gas before the gas is injected into the reactor. In order to prevent undesirable accumulation of oxidants in such processes it is essential to use a relatively expensive and more complicated reforming unit which must be especially designed and constructed to operate efficiently under relatively severe and narrow operating conditions. Such a reforming unit can best be described as a "stoichiometric reformer" in which the gas reformed therein has a low concentration of oxidants. In other words, the reformer outlet gas is suitable for direct use as a reducing gas without having to cool the gas to remove water prior to injection into the reactor. In order to prevent the accumulation of excess amounts of oxidants in the reformed gas and to avoid undesirable carbon deposition on the catalyst, the stoichiometric reformer must be operated at a relatively high temperature in the range of about 900.degree. C., or higher, which operating temperature is substantially higher than that required for conventional or "non-stoichiometric" reforming units. The overall capital cost of a stoichiometric reforming unit is considerably higher than for a conventional reformer since the materials of construction must be extremely heat resistant. Additionally, when operating at such higher temperatures, special steps must be taken to insure that a high temperature resistant catalyst is used further complicating and increasing the overall cost of the reforming unit.